Not Applicable.
Not Applicable.
The field of process chromatography is concerned with analyzing gas samples flowing through a process pipeline. A sample from a gas pipeline may be taken by use of a sample probe or other sampling device, which then provides the sample to a gas chromatograph. The gas chromatograph separates the sample into its individual components, using a variety of detectors to analyze the concentration of the resulting component bands in the sample. In the oil and gas industry, the knowledge of what fluid is being transported by the pipeline is useful for a variety of purposes, such as source identification and custody transfer.
FIG. 1 shows a known gas chromatograph system (not to scale). Gas flows through a process pipeline 110, a sample of which is taken by a sample probe 120 prior to being introduced to gas chromatograph (GC) 100. The gas sample may be filtered and heat traced generally along tubing 130 before flowing into gas chromatograph 100. Heating may be required for gases that may condense into a part gas, part liquid flow at cooler temperatures. After being analyzed by the gas chromatograph, the gas sample is either returned into the process pipeline 110, or vented to the atmosphere. As used herein, the term gas chromatograph is being used in its broad sense, to include what is traditionally known both as the sample handling system and as the carrier pre-heat system.
Referring to FIG. 2A, gas chromatograph 100 includes valve assembly 210 connected to multiple columns 220 and detectors 230, in this case, thermal conductivity detectors (TCD""s). A gas sample generally follows path 240 through valve assembly 210, columns 220 and TCD""s 230. The valve assembly allows the selection of columns 220 which contain a liquid phase, or porous polymer, or other material. Two types of columns are packed columns and capillary columns. Referring now to FIGS. 2B and 2C, packed columns 220 are filled with a liquid coated solid support or porous polymer. Capillary columns 220 are coated on their interior with a liquid or porous polymer. In either case, the polymer on the inside of the column acts to separate the gas sample into multiple fractions, each fraction that is to be analyzed being sequentially directed to the TCDs (or other detectors) 230. For example, a gas sample may contain various molecular weight hydrocarbon components such as ethane, methane, and heavier molecules. Ideally, each of these components would be analyzed individually. The resulting analysis could be normalized to minimize the effects of varying sample size from one injection to the next. In general, column 220 separates the gas sample so that more volatile components would elute from the column first, followed by less volatile components (although the use of valve switching may cause the components not to elute at the detector in that order).
Referring to FIGS. 3A and 3B, the operation of a sample valve is shown. Valve 300 includes a plurality of valve ports, labeled 1-6. Incoming line 310 provides a gas sample to valve 300. Exhaust line 320 expels the gas sample from the valve 300. Solid lines 330 show open passages between ports, whereas dotted lines 340 indicate blocked passages between the ports.
A solenoid (not shown) places valve 300 into either an ON position, as shown in FIG. 3A, or an OFF position, as shown in FIG. 3B. When a valve is in the ON position, sample gas flows from incoming line 310, through port 1 to port 6, through line 315 and finally through port 3 to port 2 and out exhaust line 320. When the valve is in the OFF position, sample gas flows from incoming line 310, through port 1 to port 2 and out through exhaust line 320. At the same time, carrier gas flows through port 5 to port 6 into line 315 where it displaces the sample gas. The carrier gas then flows from port 3 to port 4 and injects the sample onto the column. Of course, the designation of OFF versus ON is somewhat arbitrary and the opposite nomenclature could also be used.
FIG. 3C illustrates how a pair of valves may operate either alone or in combination with additional valves (not shown). A first valve 300 includes an array of six valve ports. A second valve 350 also includes an array of six valve ports. Associated tubing 310, 315, 320, 325 and 390, and columns 360 and 370 are also shown as well as dual TCD""s 380.
Incoming line 310 is attached to a sample transport line (not shown). When first valve 300 is in an OFF position, gas sample flows from incoming line 310 to port 1 to port 2 of the valve 300 and out exhaust line 320. When valve 300 is in an ON position, however, gas sample flows from port 1 to port 6 and then through sample loop 315. That gas then flows from port 3 to port 2 of valve 300 and is expelled out exhaust line 320. At this time, the sample loop 315 is filled with a gas sample. This means that, if valve 300 is turned OFF at this time, a gas sample is trapped within the sample loop 315.
Turning now to valve 350, when it is in an OFF configuration, carrier gas flows from carrier gas input line 390 through port 2 of valve 350, to port 1 and then through carrier tubing 325. At this time, valve 300 is also in an OFF configuration, so that the carrier gas in tubing 325 is forced through port 5 to port 6 and through gas sample tubing 315. Consequently, this action forces the gas sample down column 360 via ports 3 and 4. The gas sample can then additionally be forced through column 370 and into the dual TCD 380 via ports 4 and 3. Thus, the valves may be connected in series to form xe2x80x9cchannels.xe2x80x9d Each channel feeds into a corresponding thermistor pair (a measurement thermistor and a reference thermistor), which measures the amount of a component in the process sample. Alternatively, downstream analyzer valves can be arranged in the system to select a desired column or detector. The graph on which the data are presented has a series of peaks corresponding to the detected components (such as ethane, methane, etc.), and is generally referred to as a chromatogram.
FIG. 4 illustrates a simplified gas chromatograph 400 as is broadly known in the art. Sample valve 410 connects to sample-in line 420, sample out line 430, carrier-in line 440 and column line 450. Sample-in line 420 connects to sample shut-off valve 470 upstream of the sample valve 410. Immediately upstream of sample shut off, sample in line 420 connects to a sample pre-heat coil. Further upstream, sample-in line 420 connects to, e.g., a process pipeline (not shown). Downstream of the sample valve 410, column line 450 connects to column 460. Column 460, in turn, connects downstream to the remainder of the gas chromatograph, including TCD 480, with measurement line 481 and reference line 482.
During operation, a sample of fluid is delivered from a process pipeline or similar source through sample-in line 420. Once the sample is inside the sample valve 410, sample shut off valve 470 is actuated, closing off sample valve 410 from the upstream sample source. At this time, the sample in the sample valve 410 is allowed to equilibrate with atmospheric pressure by exhausting or bleeding the excess sample through sample out line 430. The sample valve 410 then actuates, changing the internal flow of the sample valve 410. Carrier-in line 440, holding pressurized carrier gas, such as helium, hydrogen, nitrogen or argon, is now in communication with the sample trapped in the sample valve 410. This carrier gas displaces the sample out column line 450 and to column 460.
In process chromatography, temperature control is one of the most important characteristics of analytical performance. For example, column temperature has a dramatic effect on the retention time of the sample inside the column. As a general rule, a 30xc2x0 C. decrease in column temperature will double the retention time for a component with a boiling temperature of 227xc2x0 C. Consequently, each column of a gas chromatograph is heated to an elevated temperature. This may be accomplished by a variety of known devices or techniques. For example, as shown in FIG. 5A, a housing 500 surrounds the column (not shown in FIG. 5A) and includes a fan 510 that forces heated air to the area around the column and warms it. Another method, as shown in FIG. 5B, is to plate the column 520 with gold or other suitable substance and attach electrodes 525 to the ends of the column 520. The column exterior then heats resistively upon electrical stimulation of the electrodes.
In an attempt to improve the analytic response of the columns, an operator my engage in a program of heating and cooling the columns to various temperatures. FIG. 6 illustrates a temperature versus time graph for a xe2x80x9ctemperature programxe2x80x9d. As an example, the effect that temperature programming has on component retention times can be illustrated.
By way of explanation, FIG. 7 shows an example of a chromatogram. As various molecules elute from the columns 460 based upon their volatility, they are measured by a concentration-dependent detector such as a thermal conductivity detector (TCD), a flame photometric detector (FPD), a photoionization detector (PID), a helium ionization detector (HID), or an electrolytic detector. The measured values appear on the chromatogram as a series of peaks. The peak maximum corresponds to the absolute retention time (i.e. time elapsed from injection of sample) for each component in the gas chromatograph system, with the area under each peak being related to the concentration of that component in the sample. To operate the system efficiently, the valve switching directs the samples from column to column at predetermined times. The columns are sized to provide adequate time between critical components (i.e. for valve switches).
In laboratory applications, temperature programming is used to shorten the analysis times of heavier samples while improving detection limits through the reduction of xe2x80x9cband spreadingxe2x80x9d. Band spreading is the phenomenon where a component curve on a chromatogram becomes spread out and less distinct. FIG. 9 (not to scale) shows the effects of band spreading on a simplified chromatogram.
In FIG. 9, curve 901 is a chromatogram without band spreading, while curve 902 is the corresponding curve with band spreading. The term t represents time, tr is retention time, h is height, Wb indicates the width at the base of the curve, W0.5 represents the width of the curve at half-height, Wi is the width of the curve at the inflection point, and 0.607 h shows the height of the curve at the inflection point. With band spreading, it is more difficult to identify these points accurately. Further, if the band curve becomes spread beyond the desired switching time, a portion of the curve would not be measured by the chromatograph. Alternately, the valve switching time could be delayed for the elution of the component but this would lead to longer analysis times. It is important to have short analysis times in process chromatography to provide good process control. Thus, excessive band spreading results in measurement errors or longer analysis times.
With a linear temperature program rate, the spacing between members of a homologous series is linear rather than logarithmic and the peak widths are nearly constant. For example, FIG. 8A shows a gas chromatogram for an isothermal (i.e. constant temperature) column. FIG. 8B illustrates the same gas sample analyzed with a temperature programmed column.
One problem with temperature programming is that there exists a time lag between heating the exterior of the column and the heating of the interior of the column (where the sample is). Consequently, the program must be adjusted and timed to ensure that the inner portion of the column is at the correct temperature. Another problem with temperature programming is the trade-off between a decrease in analysis time and the cooling time required to achieve the starting temperature. In other words, for process (on-line) applications, the problem is even more complicated, because the laboratory techniques used to shorten the cool-down time such as cryogenic (liquid nitrogen) cooling aren""t practical for process (on-line) applications. The vortex chillers used in process chromatographs require high-pressure ( greater than 100 psig) instrument air for optimum efficiency. Unfortunately, many field locations don""t have high-pressure instrument air available.
In addition, if the temperature program is not highly reproducible, then where two components elute very close in time, their position on the gas chromatogram could be switched. For example, the retention of highly branched isomers could be transposed with only slight variations in temperature. This could result in components being mis-identified.
Further, even where the column is fully heated to the correct temperature, of xe2x80x9cband spreadingxe2x80x9d can still result. The problem of band spreading arises in part from the heating of the sample and carrier gas as they move through the column. The sample and carrier gas are at a lower temperature than the column as the sample and carrier gas enter the column. But gradually, the sample and carrier gas are heated by the surrounding column, decompressing and accelerating to a higher velocity. As a result of the decompression of the sample and carrier streams in the column, most of the separation of components in the sample is completed at the front of the column. In a 60-meter capillary column, a majority of the separation might occur in the first few meters of the column.
Historically, chromatograph research has focused on developing small diameter capillary columns to compensate for this problem. However, this solution has been unsatisfactory because the complexity of the gas chromatograph varies directly with column diameter and the reliability varies inversely. Thus, gas chromatographs with very small column diameter (i.e.  less than 0.25 mm inner diameter) are impractical for process (on-line) applications.
Another contribution to band spreading is the kinetic rate of transfer of sample molecules between the mobile (carrier gas) and stationary (liquid) phases. The equilibrium between the two phases is established so slowly that the column always operates under nonequilibrium conditions. Since the diffusion coefficient varies inversely with temperature (i.e. the column efficiency varies directly with temperature), the component retention time shifts earlier when the temperature is increased. Likewise, the retention time shifts later when the temperature is decreased.
Other problems with the arrangement of FIG. 4 also exist. Another problem is xe2x80x9cretention time driftxe2x80x9d that arises from fluctuations in temperature of the carrier gas. Thus, where there is retention time drift, the entire curve might shift to the right or the left. This is a problem because where the component peaks overlap or extend beyond the switching time for a corresponding analyzer valve, the offending portion of the curve is not measured by the chromatograph.
In process chromatography, it is important to have short analysis times to provide sufficient analytical feedback for process control. For this reason, the process chromatographer sets the switching times as close together as realistically possible to provide the fastest possible chromatograph, and so merely allowing more component separation (i.e. longer analysis times) is not a best-case solution.
It has been believed to be desirable, therefore, to control the inlet carrier gas at a temperature optimized for the gas chromatograph temperature, usually chosen in the range of 80-85xc2x0 C. with little variation. It has been difficult to heat the inlet gas to a consistent temperature, however. One effort involved placing a length of tubing inside a heated zone, while at the same time, coiling the tubing in a compressed corkscrew manner to conserve space. However, even heating of very long coils of tubing, such as 50-foot coils, does not reliably heat the inlet gas to the desired temperature. This is due to the fact that the ambient temperature of a process gas chromatograph varies from xe2x88x9218 to 55xc2x0 C. For this reason, the resulting temperature of the inlet gas should be monitored using a Platinum resistance thermal detector (RTD) inserted into the gas stream.
A related problem is variation in component retention time arising from fluctuations in the inlet carrier pressure. Since inlet pressure fluctuations affect the carrier flow rate, they also result in retention time drift. It is desirable therefore to eliminate or minimize these variations in inlet carrier pressure.
As can be seen, a number of problems exist with current gas chromatographs and a gas chromatograph is needed that solves these and other problems. The ideal process gas chromatograph would be both fast and accurate, eliminating or severely reducing many of the measurement errors known in the prior art. It would also be simple and inexpensive to manufacture. In a perfect world, the device or method that solves these problems would do so on its own, requiring little human supervision or maintenance. It would also have considerable longevity, including being sturdy and not prone to breakage.
One embodiment of the invention is a gas chromatograph including a column to separate components of a fluid sample, a valve switch connected upstream of the column and downstream of sample and carrier gas sources, and first and second heaters for heating the column and carrier gas stream, respectively. The carrier gas stream is heated to one or more temperatures higher than the temperature of the column.